The invention concerns an electrode unit for rechargeable electrochemical cells, e.g. accumulator cells, whose energy storage properties are drawn from the deposition of an element such as metal or an alloy, and a rechargeable electrochemical cell equipped with at least one such electrode unit.
Electrochemical cells are current cells which can convert chemical energy into electric energy. If such a conversion is reversible, i.e. if such an electrochemical cell can be recharged by a current opposite to the discharge current, then this cell is called an accumulator cell.
Rechargeable electrochemical cells (accumulator cells) which draw their energy storage properties from deposition of an element as a metal or an alloy, differ from conventional accumulator cells in that the mass storing negative electrochemical energy is deposited on the negative electrode during charging of the cells. In contrast thereto, in conventional cells, e.g. Ni/Cd or Pb/PbO2, a substance is provided in the negative electrode which is chemically converted during charging and transferred into a higher energetic state to thereby store energy.
The principle of a rechargeable electrochemical cell which gains its energy storage properties from deposition of an element as a metal or an alloy is illustrated below with the example of a LiCoO2 cell.
The rechargeable electrochemical cell is assembled in the discharged state and consists, at this time, of an electrode which is positive during charging and contains LiCoO2 as electrochemically active intercalation material. The entire lithium is in the positive electrode. The electrode which is negative during charging consists initially of a current collector comprising a discharge conductor which may e.g. consist of nickel sheet metal or another electron-conducting material. The positive and negative electrodes are segregated by a separator. The continuous pore matrix of the separator and all remaining spaces between the LiCoO2 crystallites and the current collector of the negative electrode are filled with an electrolyte which conducts Li ions. During charging, some of the CO3+, ions are oxidized in the LiCoO2 into Co4+ ions, i.e. electrons are discharged to the current collector of the positive electrode. Same are transported through an external electron conductor (charging device) to the negative electrode. At the same time, for charge compensation, Li+ ions are deintercalated in the LiCoO2 crystallite, i.e. the lithium ions leave the crystal grid and move through the electrolyte towards the negative electrode where they are deposited in metallic form on the current collector while accepting one electron each. Alternatively, this process can be carried out by forming an alloy if the current collector consists of an alloy-forming material. In both cases, during metallic deposition and formation of alloys, larger volume increase occurs on the side of the negative electrode. The deposition (deintercalation) of the lithium ions also produces a volume increase at the positive electrode.
The terminology for utilization of electric energy refers to the “utilization process” as the discharging process of a battery or an accumulator. The positive electrode is thereby referred to as the cathode, and the negative electrode as the anode. Discussion herein mainly concerns the charging process, wherein oxidation or reduction of the electrodes are reversed compared to the discharge process. During charging, the anode is the positive electrode of intercalation material and the cathode is the negative electrode where metal is deposited or an alloy is formed during charging. Since the charging process is primarily discussed below, the positive electrode is the anode and the negative electrode is the cathode.
Storage of electric energy through deposition of a light element (e.g. lithium) or formation of specific light alloys (e.g. LiAl) entails high gravimetric energy densities of the negative electrode which produces, however, large volume changes of the negative electrode during charging or discharging.
The volume work on the cathode side and therefore the mechanical pressure on a separator, usually disposed between anode and cathode, and on the battery housing is produced through deposition of the metal or through formation of alloys with the metal of the electrode of the current collector. This occurs primarily at the edges of the electrode, having the highest current density.
The metal is often deposited in the form of fine needles (dendrites or whiskers) and therefore has a sponge-like morphology. Consequently, much more space is required than that theoretically calculated. The dendritic deposition on the negative electrode (cathode) may cause a short-circuit as soon as it develops around or through the separator disposed between the electrodes, or the separator can no longer withstand the mechanical load.
On the side of the positive electrode (anode), the use of intercalation materials during dislocation (deintercalation) of the metal ions weakens the overall binding of the ions in the host grid which usually also increases the volume.
Usually, units having a positive electrode/separator/negative electrode are produced and combined, in dependence on the requirements, to form the battery. For prismatic cells, several units are stacked on top of one another and each of the current dischargers of the anodes and cathodes are connected. For round cells, an elongated unit is rolled-up. These packets or stacks are then disposed in a housing which should tightly hold the packet to prevent displacement of the electrodes with respect to one another and the inherent risk of a short-circuit. When filling the electrolyte, penetration of the electrolyte into the pores of the battery component, which optionally swell, produces high pressures and the positive electrode firmly abuts the separator which, in turn, firmly abuts the negative electrode.
In conventional systems without considerable volume change during electrochemical activity, this is a desired effect. In the relevant systems showing large volume changes, this construction can cause a short-circuit when either the separator cannot withstand the mechanical load and breaks, or when the metal deposited e.g. in the form of dendrites or whiskers or the alloy formed on the side of the cathode grows through the separator and/or when the deposited metal or formed alloy develops around the separator from the electrode edge to pass from the anode to the cathode. In any case, such a volume change can cause deformation of the battery housing.
EP 0 766 326 A1 describes an electrode unit of the kind categorizing the invention comprising a ceramic or glassy substance disposed onto the electrode surface and formed by annealing into a continuous fine-pored separator layer.
Disadvantageously, when charging an accumulator cell provided with such electrodes, the mass deposited on the negative electrode can penetrate through the porous separator and cause a short-circuit. Moreover, deposition of the mass during charging of the accumulator cell entails a considerable volume increase of the electrode such that in an electrode unit of this design, there is the danger of failure of the separator and/or deformation of the accumulator housing.
It is therefore the underlying purpose of the present invention to further develop an electrode unit of the above-mentioned type such that the mechanical pressure produced by the volume change of the electrodes is accommodated and short-circuits are reliably prevented.
To achieve this object, the invention provides an electrode unit of the above-mentioned type having an electrode whose volume increases during charging through metal deposition or alloy formation, comprising a porous separator substantially completely surrounding the electrode, and an electrically insulating spacer which covers at least part of the electrode surface and has spaces accommodating the volume increase.